This invention relates to a process for producing stabilized polyolefins by inactivating residual catalyst in the polymer.
Polyolefins are produced from olefins such as ethylene, propylene and the like by polymerizing the same usually in the presence of a catalyst prepared from a halide of a transition metal such as titanium, vanadium, zirconium and the like and an organometallic comound such as alkyl aluminum, alkyl aluminum halide and the like.
The polymerization catalyst of this type, in particular, the transition metal halide component contained therein is insoluble to an inert hydrocarbon used as a diluent for the polymerization reaction and liquefied monomer present in an excess amount and most portions thereof remain in the resulting polymer. Since the remaining catalyst results in undesirable discoloration upon heat plasticization of the polyolefins and reduces the heat stability of molded products, it should be removed from the polymer as much as possible.
Methods of removing a remaining catalyst from resulted polyolefins known so far include, for example, such a process as treating the resulted polymer with an alcohol to render the remaining catalyst water soluble and then extracting the same with water or washing out the catalyst with liquid hydrocarbon. Such an alcohol treatment, however, necessitates a costly corrosion resistant material for the apparatus since hydrogen halide produced through the reaction between the remaining catalyst and the alcohol attacks the apparatus, as well as requires complicated apparatus and processes for recovering and refining the alcohol and the hydrocarbon from the washing solution containing the decomposition products of the catalyst dissolved therein. Accordingly, it is of great importance from the industrial point of view to develop a method capable of removing or inactivating catalyst residues remaining in the product polyolefins with ease.
Various types of highly active catalysts have recently been proposed so that lesser amounts of catalyst need to be used to perform the polymerization reaction while simplifying the catalyst removal process. While the polyolefins polymerized in the presence of a highly active catalyst of this type contain only a comparatively small amount of residual catalyst, they still contain a small amount of catalyst having metal-to-halogen bonds, which releases active halogen upon heat plasticization of the polymer which attacks the apparatus. Moreover, the polyolefins containing small amounts of residual catalyst significantly lack thermal stability even with the addition of a conventional stabilizer and will undesirably discolor upon heat plasticization. It is known that such discoloration is attributable to the presence of a reaction product which is formed from a transition metal halide of a lower valency and a phenolic antioxidant. The discoloration can be prevented by treating the catalyst remaining in the polymer with a compound having a specific chelating action. In short, even the polyolefins polymerized in the presence of a highly active catalyst which is present in the polymerization system in only small residual amounts have only a poor commercial value unless subjected to an inactivation treatment. It is therefore a general requirement to remove the active halogen compound and to inactivate the residual transition metal compound to prevent discoloration of the polymer during heat plasticization and to improve the heat stability of the molded products.